Self-contained downhole tools on a wire line for measuring pressure and temperature in a borehole have been in use for some time in oil field applications. Pressure and temperature measurements in a production well over a long period of time provide significant data for analysis of the productivity of the well.
Where the downhole tool uses a contained D.C. battery pack for electrical power to the downhole measuring and recording equipment, the battery pack is a limiting factor to the period of time that downhole measurements can be made because the equipment expends the electrical power continuously. Where the electrical power is utilized intermittently by a downhole switching system, other problems occur. For example, some tools utilize memory systems for recording data which automatically erase if the power is shut off thus losing prior data. In either case, it is desirable to know prior to running the tool in the well bore that the tool is functionally ready to operate, and in a downhole operation, that the data is stored independently of electrical power to the system.